You throw a throwing net.
The throwing net closely misses the lich.
Message log:
_The lich touches you but does no damage.
You throw a throwing net. The throwing net hits the lich.
_The lich is caught in the net!
You throw a throwing net.
The throwing net closely misses the lich.
_The lich completely misses you.
3 throwing nets.
You throw a throwing net. The throwing net hits the lich.
The lich is caught in the net!
The lich gestures wildly while chanting.
_The lich seems to speed up.

My nets twice hit this lich, and game said it was caught. Though there were no visual confirmation. It wasn't struggling in net and acted as so it wasn't caught.
It is the third lich I'm trying to kill with nets. First one brought the same problems, second one was successfully caught and killed. I used nets not so often in this game, maybe two times besides these liches and nets worked.

Fixed in trunk (0.16-a0-2100-g2a841a0), thanks! The problem happened when there was already a net on the floor (the first one you threw that missed): the new nets would trap the lich but then immediately merge with the floor stack, untrapping it.